As a note, it is not the GTX 970 having VRAM capacity issues there, that' the mods being too intensive and whether it's a GTX 970 or a Radeon 390 won't change that.
Also, it is a well proven fact that just because a game can use more than say 2GB or 3GB, does not mean that it has a performance impact. Give a game a GTX Titan Z with 12GB and you'll see tons of memory being used, but the game will happily function with a fraction of that memory capacity without any issues with a comparably performing setup.
Give a game more memory and it will use more, but that just means it gets lazy about removing data it isn't using anymore. If the capacity is there, then it doesn't need to remove textures and such that aren't in use. This does not impact performance, it just gives misleading VRAM usage numbers. The 970 won't see issues with capacity simply because its capacity is enough for its performance even in SLI. Just look at AMD's Fury X. The card performs on par with the GTX 980 Ti in most heavy situations, but it's only a 4GB card that is still nowhere near a VRAM capacity bottleneck and thats with only a .5GB advantage despite a significant performance difference from the 970.
Case in point, if the 970 runs into a capacity issue, then the 980 would too because it has only a small VRAM advantage.